Belt dust mass (as measured at 1.33 mm) as a function of resolved belt radius for the REASONS sample (coloured points with error bars). The filled contours represent the 2D density distributions of belts ≤ 30 (blue solid), 30–200 (green dotted), and >200 (red dashed) Myr-old. Contour lines are defined to contain 20, 50, and 80% of the population for each age range. Black lines represent predictions from a simple collisional evolution model as described in Sect. 5.3, for collisional ages of 5 Myr (left) and 5 Gyr (right), an initial dust mass of 1 M_⊕ (solid) or 0.1 M_⊕ (dashed), and dust masses evolving with time as t^−0.4 or t⁻¹ between 5 Myr and 5 Gyr.